Power oscillator

ABSTRACT

An oscillator includes a solid state active device having an input and an output, a feedback circuit connected from the output of the active device to the input of the active device, the feedback circuit providing suitable positive feedback to initiate and sustain an oscillating condition at a fundamental frequency, and a waveform modifying circuit connected to the output of the active device, wherein the waveform modifying circuit is adapted to modify the waveform on the output in a manner which increases an efficiency of the oscillator. For example, the waveform modifying circuit comprises transmission lines and optionally components which provide a high impedance for odd harmonics of the fundamental frequency. The waveform modifying circuit may further include transmission lines and optionally components which provide a low impedance for even harmonics of the fundamental frequency. The waveform modifying circuit utilize class F amplifier principles to cause the waveform on the output to be relatively more square in shape.

Certain inventions described herein were made with Government supportunder Contract No. NAS10-99037 awarded by National Aeronautics and SpaceAdministration. The Government has certain rights in those inventions.

BACKGROUND

1. Field of the Invention

The invention relates generally to oscillator circuits and morespecifically to a high power, high efficiency oscillator circuit.

2. Related Art

Class F amplifiers are well known in the art and are described invarious articles and books including “FET Power Amplifier with MaximallyFlat Waveform,”, by F. H. Raab, IEEE Trans., MTT, Vol. 45, No. 11, Nov.1997, pp. 2007-2011, and

RF Power Amplifiers for Wireless Communications, by S. C. Cripps, S. C.,Norwood, M A: Artech, 1999, ISBN 0-89006-989-1. The state of the art forsuch amplifiers is believed to be about 50 watts and 65% efficient atsomewhat less than 300 MHz frequency of operation.

The inventors are unaware of any reference which describes a poweroscillator which utilizes class F amplifier principles. Class Famplifiers typically operate at a frequency range of less than 300 MHzand use lumped multiple-resonator output filters to control the harmoniccontent of their drain-voltage and/or drain-current waveforms. With anunstable load, these conventional Class F tank circuits develop highvoltages which would be potentially destructive if fed back to the input(e.g. gate) of a solid state active device. Accordingly, the well knownClass F drain circuits are not well suited for an oscillatorconfiguration, particularly at high power and dynamically varying loadconditions, such as those presented by an electrodeless lamp.

SUMMARY

The following and other objects, aspects, advantages, and/or features ofthe invention described herein are achieved individually and incombination. The invention should not be construed as requiring two ormore of such features unless expressly recited in a particular claim.

In general, the present invention relates to the type of poweroscillators described in PCT Publication Nos. WO 99/36940 and WO01/03161, each of which is herein incorporated by reference in itsentirety. The power oscillators described in these references arebelieved to define the state of the art in terms of power output,frequency range, and efficiency.

According to one aspect of the invention, an oscillator includes a solidstate active device having an input and an output; and a feedbackcircuit connected from the output of the active device to the input ofthe active device, the feedback circuit providing suitable positivefeedback to initiate and sustain an oscillating condition at afundamental frequency; characterized in that the oscillator furtherincludes a waveform modifying circuit connected to the output of theactive device, wherein the waveform modifying circuit is adapted tomodify the waveform on the output in a manner which increases anefficiency of the oscillator. For example, the waveform modifyingcircuit comprises transmission lines and optionally components whichprovide a high impedance for odd harmonics of the fundamental frequency.The waveform modifying circuit may further include transmission linesand optionally components which provide a low impedance for evenharmonics of the fundamental frequency.

According to another aspect of the invention, an oscillator includes asolid state active device having an input and an output; and a feedbackcircuit connected from the output of the active device to the input ofthe active device, the feedback circuit providing suitable positivefeedback to initiate and sustain an oscillating condition at afundamental frequency; characterized in that the oscillator furtherincludes a transmission line coupled to the output of the active device,the transmission line having an effective electrical length of onetwelfth of a wavelength of the fundamental frequency. The oscillator mayfurther include another transmission line connected to the output of theactive device and having an effective electrical length of one eighth ofthe wavelength of the fundamental frequency.

According to another aspect of the invention, a high power solid statepower RF oscillator includes a power FET having a gate, a drain, and asource, wherein the source is RF grounded and suitable voltages arerespectively applied to the drain and gate to place the transistor in aconducting condition; a drain circuit connected to the drain of thepower FET; and a feedback circuit connected between the drain circuitand the gate, wherein the feedback circuit is adapted to providesuitable positive feedback to initiate and sustain an oscillatingcondition with a fundamental frequency being greater than 300 MHz andwith an RF power output in excess of 50 watts; characterized in that thedrain circuit comprises transmission lines having widths which provide aconjugate match for the drain at a fundamental frequency and lengthswhich provide a high impedance for odd harmonics of the fundamentalfrequency. The drain circuit may further include a transmission linehaving a length which provides a low impedance for even harmonics of thefundamental frequency. For example, the drain circuit comprises a firsttransmission line connected at a first end to the drain and at a secondend to an output matching circuit, and a second transmission line whichis an open stub connected to the second end of the first transmissionline, wherein the first transmission line and second transmission lineseach have a width which provides a conjugate match for the drain at afundamental frequency and an effective electrical length of about onetwelfth of a wavelength of the fundamental frequency. The drain circuitmay further include a third transmission line connected at one end tothe drain and having an effective electrical length of one eighth of thewavelength of the fundamental frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments as illustrated in the accompanyingdrawings, in which reference characters generally refer to the sameparts throughout the various views. The drawings are not necessarily toscale, the emphasis instead being placed upon illustrating theprinciples of the invention.

FIG. 1 is a first block diagram of an oscillator according to thepresent invention.

FIG. 2 is a second block diagram of an oscillator according to thepresent invention.

FIG. 3 is a schematic diagram of a first example of an oscillatoraccording to the present invention.

FIG. 4 is a printed circuit board diagram for the first example.

FIG. 5 is an assembly level drawing for the first example.

FIG. 6 is a schematic diagram of a tuning circuit suitable for use withthe oscillator of the present invention.

FIG. 7 is a schematic diagram of a bias circuit suitable for use withthe present invention.

FIG. 8 is a printed circuit board diagram for a second example of anoscillator according to the present invention.

FIG. 9 is an assembly level diagram of the second example.

FIG. 10 is a combined graph of power versus drain voltage and efficiencyversus drain voltage for one configuration of the second example.

FIG. 11 is a combined graph of power versus drain voltage and efficiencyversus drain voltage for another configuration of the second example.

FIG. 12 is a block diagram of a conventional RF system.

FIG. 13 is a block diagram of an RF system including a protectioncircuit in accordance with a present aspect of the invention.

FIG. 14 is a schematic diagram of a third example of an oscillatoraccording to the present invention, including a protection circuit.

FIG. 15 is a schematic diagram of a protection circuit in accordancewith the present aspect of the invention.

FIG. 16 is a printed circuit board diagram for the third example.

FIG. 17 is an enlarged, fragmented assembly level diagram of the thirdexample.

FIG. 18 is an assembly level diagram of a fourth example of anoscillator including the protection circuit.

FIG. 19 is a combined graph of power versus drain voltage and efficiencyversus drain voltage for the fourth example.

FIG. 20 is a printed circuit board layout for a fifth example of anoscillator according to the present invention, including an optionaldirectional coupler circuit.

FIG. 21 is an assembly level diagram of the fifth example.

FIG. 22 is a combined graph of power versus drain voltage and efficiencyversus drain voltage for the fifth example.

FIG. 23 is a graph of frequency of operation versus control voltage forthe fifth example.

DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth such as particularstructures, interfaces, techniques, etc. in order to provide a thoroughunderstanding of the various aspects of the invention. However, it willbe apparent to those skilled in the art having the benefit of thepresent disclosure that the various aspects of the invention may bepracticed in other examples that depart from these specific details. Incertain instances, descriptions of well known devices, circuits, andmethods are omitted so as not to obscure the description of the presentinvention with unnecessary detail.

With reference to FIG. 1, an oscillator includes an active device 3(e.g. a solid state transistor) with an input and an output. The outputof the active device 3 is connected to an impedance matching circuit 5.A signal on the output of the active device 3 is fed back to the inputof the active device 3 by a feedback circuit 7. Preferably, as shown inFIG. 1, the feedback signal is taken from the impedance matching circuit5. The feedback circuit 7 provides suitable positive feedback toinitiate and sustain an oscillating condition. Preferably, the feedbackcircuit 7 is further configured to protect the active device 3 fromdestructive feedback, as is described in detail in the aforementioned'940 publication. In accordance with the present invention, theoscillator further includes a waveform modifying circuit 9 connected tothe output of the active device 3 which is adapted to modify thewaveform on the output of the active device 3 to improve an efficiencyof the oscillator.

With reference to FIG. 2, an oscillator is similarly configured asdescribed above in connection with FIG. 1, and further includes a tuningcircuit 11 for adjusting an operating frequency of the oscillator and abias circuit 13 for providing suitable bias and operating voltages tothe oscillator.

With respect to the power oscillators described in the '940 and '161publications, the circuit connected to the output of the active deviceis adapted primarily for matching and feedback, and no waveform shapingtakes place at the output.

For a power MOSFET transistor in an amplifier configuration, efficiencymay be improved by impressing a square wave on the gate and/or thedrain. A square waveform reduces the amount of power dissipation in thetransistor, thereby increasing efficiency.

Although various of the principles of class F amplifier operation arewell known, it is a relatively complex task to combine those principleswith other circuit requirements necessary to achieve a high power, highfrequency, high efficiency oscillator. For example, those otherrequirements include providing a conjugate match for the drain outputfor power transfer at a given frequency, transformation of the impedanceat the drain to match a nominal load impedance, and providing a suitablefeedback signal. The inventors are unaware of any reference orreferences, alone or in combination, which provide sufficient guidancefor constructing a high power, high efficiency oscillator which utilizesclass F amplifier principles to increase efficiency while satisfying allof the other circuit requirements necessary to drive a complex load suchas an electrodeless lamp.

In accordance with one aspect of the present invention, a relativelymore square waveform is provided to the drain of a transistor in anoscillator configuration using only micro-strip transmission lines,stubs, and/or non-inductive discrete components to 1) provide highimpedance for odd harmonics to reflect the odd harmonics back to thedrain; and, optionally if needed, 2) provide low impedance for evenharmonics to suppress reflection of the even harmonics back to thedrain. The reflected odd harmonics are combined with the fundamentalfrequency at the drain to provide a waveform with a more square shape ascompared to the sinusoidal waveform of fundamental frequency alone.

FIRST EXAMPLE

With reference to FIGS. 3-5, an oscillator circuit includes an activedevice Q1, such as a power LDMOS transistor. A source S of thetransistor Q1 is grounded and a gate G of the transistor provides aninput terminal and a drain D of the transistor provides an outputterminal. A suitable DC voltage Vdss is applied to the drain D through atransmission line TL3. A suitable DC voltage Vgs is applied to the gateG through a resistor R1 and transmission lines TL7 and TL8. The drain Dis connected to a drain circuit which includes a transmission line TL1connected at one end to the drain D and connected at the other end to ajunction of two transmission lines TL2 and TL4. The transmission lineTL2 is open (unconnected) on its other end. The transmission line TL3 isconnected on one end to the drain D and is connected to ground through acapacitor C2 on the other end. The transmission line TL3 is also used asa DC line to provide the DC voltage Vdss to the drain D.

The transmission line TL4 has an open circuit at one end and isconnected at a selected point along its length to a feedback circuitwhich includes a first feedback capacitor Cf1, a transmission line TL11,and a second feedback capacitor Cf2 connected in series. The capacitorCf2 is connected to the junction of the transmission lines TL7 and TL8.Two matching stubs TL5 and TL6 are connected at respective ends to thejunction of TL7 and the gate G. Two open end tuning stubs TL9 and TL10are each connected at one end to the junction of the transmission lineTL8 and the resistor R1. A variable tuning capacitor C4 is connectedbetween the transmission line TL10 and ground. Filter capacitors C1 andC3 are respectively connected between the DC voltages Vgs and Vdss andground.

The drain circuit includes an output matching circuit which includestransmission lines TL1, TL2, TL3, and TL4 which are advantageouslyconfigured to simultaneously provide several matching conditions. First,a width of TL1, TL2, and TL4 is selected to provide a conjugate matchfor the drain impedance for good power transfer. Second, a length of TL1and TL2 is selected to present a high impedance for third harmonics ofthe fundamental frequency. Third, a length of TL3 is selected to presenta low impedance for even harmonics of the fundamental frequency.

An input matching circuit includes transmission lines TL5 through TL7. Atuning circuit includes transmission lines TL8 through TL10 and thecapacitor C4. A bias circuit includes the resistor R1, the filter capsC1 and C3, and their respective DC connections to the gate G and drainD. Further details regarding suitable tuning and bias circuits may behad by reference to the '940 and '161 publications.

The feedback circuit includes the capacitor Cf1 which couples a signalfrom the transmission line TL4 to the feedback transmission line TL11.The capacitor Cf2 is selected to adjust the phase of the feedbacksignal. The capacitor Cf1 and Cf2 also perform a voltage divisionfunction which reduces the feedback voltage to safe operating levels forthe gate G of the transistor Q1.

With reference to FIGS. 4-5, a suitable printed circuit board layout andassembly detail is shown for an oscillator according to the firstexample. An example methodology in accordance with the present inventionfor providing all of the necessary circuit conditions for a highfrequency, high efficiency power oscillator is as follows:

The transmission line TL2 is an open stub. The effective electricallength of the lines TL1 and TL2 is selected to be:

L _(TL1) =L _(TL2)=λ₃/4  Eq. (1)

where λ₃ is the wavelength of third harmonic of the fundamentalfrequency. The effective electrical length of each of TL1 and TL2 isapproximately equal to one twelfth ({fraction (1/12)}th) of thewavelength of the fundamental frequency. For micro-strip transmissionlines, the precise length may differ from the ideal value due tofrequency dependent variations in the effective dielectric constant ofthe substrate material. Advantageously, the matching condition for thethird harmonic is broken up into two quarter wavelength stubs. At thejunction of the lines TL1 and TL2, the third harmonic is presented witha low impedance due to the quarter wavelength length of TL2.Accordingly, little third harmonic content is passed on the output or tothe feedback stub TL4. However, TL1 and TL2 together form the desiredresonant structure for presenting a high impedance for the thirdharmonic and reflecting the signal back on the drain.

If needed, the transmission line TL3 is a short stub at the secondharmonic of the fundamental frequency. The effective electrical lengthof the line TL3 is selected to be:

L _(TL3)=λ₂/4  Eq. (2)

where λ₂ is the wavelength of the second harmonic of the fundamentalfrequency. The effective electrical length of TL3 is approximately equalto one eighth (⅛th) of the wavelength of the fundamental frequency. Theactual length of TL3 may be reduced by utilizing a capacitor (e.g. C2)to ground.

The widths of the transmission lines TL1-TL4, the length of thetransmission line TL4, and the match circuit are selected to provide aconjugate match load for the drain impedance. The selected widthprimarily effects the impedance of the line, with wider lines providinglower impedance. However, the width also affects the effectivedielectric constant and consequently the guide wavelength λ_(g).Accordingly, a trade off may occur between a desired width for the linesTL1, TL2, and TL4 and the appropriate lengths to provide the desiredcircuit conditions from #1 above. The match circuit includestransmission lines and components (e.g. TL12, TL13, C5, C6 in FIG. 5) toprovide an impedance transformation from the nominal load (e.g. 50 ohms)to the junction of TL1, TL2, and TL4 to provide a matched drainimpedance to the nominal load impedance at the fundamental frequencyover some bandwidth.

An appropriate length for transmission line TL11 and suitable values anpositions for the capacitors Cf1 and Cf2 are selected to provideappropriate feedback gain and phase shift to initiate and sustainoscillation at the fundamental frequency. Details regarding theselection of appropriate values may be had with reference to '940publication. The position of Cf1 along TL1 or TL4 may be adjusted toadjust the frequency and also to adjust the amount of feedback signalcoupled through Cf1.

SECOND EXAMPLE

With reference to FIGS. 6-9, a second example of an oscillator includesan electronically variable tuning circuit and an alternative biascircuit arrangement. The tuning circuit shown in FIG. 6 is configured toprovide a variable damping coefficient on the feedback signal to thetransistor, thereby influencing the oscillator operating frequency. Byadjusting the voltage Vf, the amount of influence the tuning circuit hason the oscillator frequency is likewise varied. In the present example,the voltage Vf determines the amount of current flowing through the PINdiodes and accordingly changes the PIN diodes conductivity. Theconductivity of the PIN diodes affects the impedance of C6, D1 and C7,D2, respectively connected between TL9/TL10 and ground, and consequentlychanges the operating frequency of the oscillator. The voltage Vf may beprovided by an external control circuit, as further described in the'940 and '161 publications. The PIN diodes in the tuning circuit may beconnected to a grounded heatsink post as described in the '161publication. The bias circuit shown in FIG. 7 is configured to receive asingle DC voltage Vdss which is provided to the drain through an RFchoke circuit and which is also divided through a resistor network toprovide a suitable voltage level for the gate bias voltage.

In practice, depending on the particular circuit configuration, anoscillator circuit may not produce significant levels of secondharmonics. In such a configuration, a shorted quarter wavelength line(e.g. the transmission line TL3) may not be needed and simple RF chokemay be utilized. With reference to FIGS. 8-9, a printed circuit boardlayout and assembly detail is shown which incorporates theabove-described tuning and bias circuits and omits the shunt for evenharmonics (TL3). Board dimensions for a fundamental frequency ofapproximately 430 MHz are approximately 78 mm by 176 mm and the boardmaterial has a nominal dielectric constant of 3. Suitable componentvalues are indicated in Table 1.

Reference Description Q1 RF Power FET, Spectrian URF1080 Cf1 0.4 to 2.4pF variable capacitor Cf2 33 pF capacitor C5 470 pF capacitor C6 2.7 pFcapacitor C7 2.7 pF capacitor C8 4700 pF capacitor C9 1000 pF capacitorC10 4700 pF capacitor C11 4.7 uF capacitor C12 9.1 pF capacitor C13 130pF capacitor C14 3.9 pF capacitor D1 PIN diode D2 PIN diode L1 330 nHinductor L2 330 nH inductor L3 18 AWG, 8 turn hand wound inductor R1100K ohm resistor R2 5.1K ohm resistor R3 5.1K ohm resistor R4 3.3K ohmresistor R5 1K ohm variable resistor R6 3.3K ohm resistor

With reference to FIGS. 10-11, exemplary performance data are shown forthe second example. The second example is configured to provide RF powerat about 430 MHz. For the data graphed in FIG. 10, the electronicallycontrolled tuning circuit is not connected and a single capacitorprovides the tuned oscillating frequency. Efficiency of greater than 80%is achieved at RF power levels between about 85 and 112 Watts.Efficiency above 79% is achieved over the entire power range illustratedbetween about 60 and 130 RF Watts. The data graphed in FIG. 11 includesthe electronically variable tuning circuit. The tuning circuit causes asmall loss in efficiency. However, efficiency is still quite good overthe range of powers illustrated.

THIRD EXAMPLE

A conventional oscillator system is illustrated in connection with FIG.12. An active device 15 provides an output to an output match circuit17. A feedback circuit 19 provides a feedback signal to an input matchcircuit 21 connected to an input of the active device 15. The outputsignal is passed through an isolator or circulator 23 to a load 25. Theisolator 23 protects the oscillator circuit from high reflections byredirecting such reflections to a dummy load. The isolator 23 passesoutput power through itself and absorbs part of that power, therebydecreasing system efficiency. Also, isolators typically operate in anarrow frequency band, are relatively large circuit components and arerelatively expensive.

As noted above, the oscillator circuits of the present inventionpreferably incorporate the feedback protection principles discussed inthe aforementioned '940 and '161 publications. Moreover, many of theoscillator circuits described in those references have stable operationwithout the use of isolators or circulators. However, reliability of theoscillator may be further improved in with a protection circuitconnected to the input of the active device.

With reference to FIG. 13, an oscillator according to a present aspectof the invention includes an active device 15 which provides an outputto an output match circuit 17, a feedback circuit 19 connected betweenthe output match circuit 17 and an input match circuit 21, where thefeedback circuit 19 provides a feedback signal from the output of theactive device to an input of the active device 15. The output signal isconnected directly to a load 25. The oscillator further includes aprotection circuit 27 connected to an input of the active device 15 andconfigured to protect the active device 15 from destructive feedback.

With reference to FIG. 14, an oscillator circuit is identical to thecircuit described above in connection with FIG. 3, with the addition ofa protection circuit 31 connected to the gate of the transistor Q1.

A preferred schematic diagram of the protection circuit 31 is shown inFIG. 15. The protection circuit includes two high power P-I-N diodes,two capacitors, two Zener diodes, and biasing components. A suitablefirst bias voltage +Vbias is connected to one end of a resistor R2. Theother end of R2 is connected to a cathode end of a PIN diode D1, acathode end of a Zener diode D3, and one terminal of a capacitor C7. Theother end of the capacitor C7 and the anode of the Zener diode D3 areeach connected to ground. A suitable second bias voltage −Vbias isconnected to one end of a resistor R3. The other end of R3 is connectedto an anode end of a PIN diode D2, an anode end of a Zener diode D4, andone terminal of a capacitor C8. The other end of the capacitor C8 andthe cathode of the Zener diode D4 are each connected to ground. Theanode end of the PIN diode D1 is connected to the cathode end of the PINdiode D2. In the oscillator circuit the junction of the PIN diodes D1and D2 is connected to the input of the active device.

Further details of the operation of the protection circuit may be hadwith reference to co-pending application no. PCT/US01/______, entitledVOLTAGE CLAMPING CIRCUIT, filed concurrently on an even date herewith.

With reference to FIGS. 16-17, an oscillator printed circuit boardlayout and assembly is substantially identical to the first example,with the addition of components and connections for implementing theprotection circuit from FIG. 15. The DC drain voltage Vds is used as thefirst bias voltage +Vbias. A separate DC voltage is provided for thesecond bias voltage −Vbias. Suitable component values are as follows:

Reference Description C7 470 pF capacitor C8 470 pF capacitor D1 PINdiode D2 PIN diode D3 13 V Zener diode D4 6 V Zener diode R2 3.3K ohmresistor R3 3.3K ohm resistor

FOURTH EXAMPLE

With reference to FIGS. 18-19, the fourth example is generally similarto the second example, with the addition of the above-describedprotection circuit. For an operating frequency of 430 MHz, as can beseen in FIG. 19, the novel oscillator circuit provides efficiency of upto 80% at an output power of 125 W, without the variable tuning circuit.Power of about 160 RF Watts is achieved at about 74% efficiency. Powerin excess of 170 RF Watts is achieved at better than 65% efficiency. Theprotection circuit, together with the protection provided in thefeedback circuit, protects the oscillator transistor when the loadimpedance changes from short circuit to a nominal load (e.g. 50 Ohm)with no circulator or isolator.

FIFTH EXAMPLE

With reference to FIGS. 20-23, a power oscillator according to thepresent invention is configured for operation in the region of 700 MHz.The printed circuit board includes an optional integral directioncoupler as described in the '161 publication for providing signalsrepresentative of forward and reflected power to an external RF controlcircuit. Board dimensions are approximately 80.24 mm by 164 mm and theboard material has a nominal dielectric constant of 3. Suitablecomponent values for oscillator are as follows:

Reference Description Q1 RF Power FET, Spectrian S1F90 C1 4.7 uFcapacitor C2 4700 pF capacitor C3 470 pF capacitor C4 470 pF capacitorC5 2.7 pF capacitor C6 2.7 pF capacitor C7 22 pF capacitor C8 470 pFcapacitor C9 1.5 pF capacitor C10 2 pF capacitor C11 130 pF capacitorC12 1000 pF capacitor D1 Zener diode, 6.2 V, 1.5 W (1SMA5920BT3) D2 PINdiode (M/A-COM MA4P7002F) D3 PIN diode (M/A-COM MA4P7002F) D6 Zenerdiode, 12 V, 1.5 W (1SMA5929BT3) D7 PIN diode (M/A-COM MA4P7002F) D8 PINdiode (M/A-COM MA4P7002F) L1 330 nH inductor L2 18 AWG, 9 turn handwound inductor L7 330 nH inductor R1 3.3K ohm resistor R3 5.1K ohmresistor R4 5.1K ohm resistor R5 100K ohm resistor R12 3.3K ohm resistorR13 3.3K ohm resistor VR1 1K ohm variable resistor (Spectrol 004G102TR)

Performance data for the fifth example is shown in FIG. 22, without thevariable tuning circuit. With the variable tuning circuit, efficiency isslightly lower. As is shown in FIG. 23, the oscillator can be tuned overa range of about 711 MHz to about 735 MHz by adjusting the controlvoltage on the PIN diodes D2, D3 over a range of 0 to 4.1 volts.

While the invention has been described in connection with what ispresently considered to be the preferred examples, it is to beunderstood that the invention is not limited to the disclosed examples,but on the contrary, is intended to cover various modifications andequivalent arrangements included within the spirit and scope of theinventions.

What is claimed is:
 1. A high power solid state power RF oscillator,comprising: a power FET having a gate, a drain, and a source, whereinthe source is RF grounded and suitable voltages are respectively appliedto the drain and gate to place the transistor in a conducting condition;a drain circuit connected to the drain of the power FET; and a feedbackcircuit connected between the drain circuit and the gate, wherein thefeedback circuit is adapted to provide suitable positive feedback toinitiate and sustain an oscillating condition with a fundamentalfrequency being greater than 300 MHz and with an RF power output inexcess of 50 watts; characterized in that the drain circuit comprisestransmission lines having widths which provide a conjugate match for thedrain at a fundamental frequency and lengths which provide a highimpedance for odd harmonics of the fundamental frequency.
 2. Theoscillator as recited in claim 1, wherein the drain circuit furtherincludes a transmission line having a length which provides a lowimpedance for even harmonics of the fundamental frequency.
 3. A highpower solid state power RF oscillator, comprising: a power FET having agate, a drain, and a source, wherein the source is RF grounded andsuitable voltages are respectively applied to the drain and gate toplace the transistor in a conducting condition; a drain circuitconnected to the drain of the power FET; and a feedback circuitconnected between the drain circuit and the gate, wherein the feedbackcircuit is adapted to provide suitable positive feedback to initiate andsustain an oscillating condition with a fundamental frequency beinggreater than 300 MHz and with an RF power output in excess of 50 watts;characterized in that the drain circuit comprises a first transmissionline connected at a first end to the drain and at a second end to anoutput matching circuit, and a second transmission line which is an openstub connected to the second end of the first transmission line, whereinthe first transmission line and second transmission lines each have awidth which provides a conjugate match for the drain at a fundamentalfrequency and an effective electrical length of about one twelfth of awavelength of the fundamental frequency.
 4. The oscillator as recited inclaim 3, wherein the drain circuit further includes a third transmissionline connected at one end to the drain and having an effectiveelectrical length of one eighth of the wavelength of the fundamentalfrequency.